Various methods are well known for wavelength tuning of lasers. Typically the tuning takes place in a device referred to as a line narrowing package or line narrowing module. A typical technique used for line narrowing and tuning of excimer lasers is to provide a window at the back of the discharge cavity through which a portion of the laser beam passes into the line narrowing package. There, the portion of the beam is expanded and directed to a grating which reflects a narrow selected portion of the laser's broader spectrum back into the discharge chamber where it is amplified. The laser is typically tuned by changing the angle at which the beam illuminates the grating. This may be done by adjusting the position of the grating or providing a mirror adjustment in the beam path. The adjustment of the grating position or the mirror position may be made by a mechanism which we will refer to as a wavelength adjustment mechanism. For many applications it is important that the laser not only be finely tunable but also that the wavelength of the beam be at a precise absolute value, with a very small deviation, such as for example 193.3500 nm .+-.0.0001 nm. This requires very precise calibration.
U.S. Pat. No. 5,450,207, entitled "Method and Apparatus for Calibrating a Laser Wavelength Control Mechanism," by Igor Fomenkov, assigned to the present assignee and incorporated herein by reference, describes a method for calibrating a wavelength adjustment mechanism for an excimer laser. In the '207 patent, a small portion of the light emitted by a laser is passed through a cell containing FeNe vapor, used as an absorption gas. The light exiting this vapor is then detected by a photodetector, and the intensity of the detected light is then analyzed. The FeNe vapor absorbs a portion of the laser light at a wavelength of 248.3271 nm. The laser has a tunable range between 247.9 nm to 248.7 nm. By detecting a dip in the intensity of the laser light passing through the vapor as the laser is slewed through a range of wavelengths, it is then known that the laser is tuned to a wavelength of 248.3271 nm. Such a technique may be used to calibrate the laser wavelength measuring system (hereafter called wavemeter). The wavemeter, now calibrated, may then accurately measure the wavelength of laser light at other wavelengths. Another such wavemeter is described in U.S. Pat. No. 5,025,445, assigned to the present assignee and incorporated herein by reference. Another wavemeter is described in U.S. application Ser. No. 08/780,865 (now U.S. Pat. No. 5,867,314), filed on Jan. 9, 1997, entitled Laser Wavelength Control Circuit Having Automatic DC Offset and Gain Adjustment, by Stuart Anderson, Docket No. 96-0012-01, assigned to the present assignee and incorporated herein by reference.
If the laser is used in a stepper in a wafer fabrication system, the stepper optics and the fabrication process are optimized for a specific laser wavelength. Accordingly, it is important that the laser wavelength be adjusted accurately so that a maximum amount of the laser energy occurs at the desired wavelength.
The FeNe vapor contains one absorption line within the tunable range of the laser and thus can only be used for calibration at one wavelength.
Another vapor which may be used for measuring the wavelength of a tunable ArF laser is carbon, which has an absorption line at 193.0905 nm. This absorption line is then used to calibrate a wavemeter.
It would be desirable to employ a wavelength measuring system which does not require calibration at only one wavelength. This would allow the operator to select a calibration wavelength closest to the final operating wavelength of interest. This would increase the accuracy of the wavelength measurement.